/Engine/Plugins/Runtime/MassEntity/Source/MassEntity/Public/MassEntityQuery.h
Goes through ValidArchetypes and sums up the number of entities contained in them. Note that the function is not const because calling it can result in re-caching of ValidArchetypes
the number of entities this given query would process if called "now"
int32 GetNumMatchingEntities ( [FMassEntityManager](API\Plugins\MassEntity\FMassEntityManager) & InEntityManager )